fix export related issues (#2005)

- `mangle` non-exported names
- `unused` on `export` of `function`
- `hoist_funs` on `export`
- `export default`
  - prohibit definition statements
  - parse `AST_Defun` properly
  - drop only unused class and function names


fixes #2001
fixes #2004

test/compress/issue-2001.js

@@ -1,5 +1,7 @@
 export_func_1: {
     options = {
+        hoist_funs: true,
+        toplevel: true,
         unused: true,
     }
     input: {
@@ -10,7 +12,9 @@ export_func_1: {
 
 export_func_2: {
     options = {
+        hoist_funs: true,
         side_effects: false,
+        toplevel: true,
         unused: true,
     }
     input: {
@@ -21,7 +25,9 @@ export_func_2: {
 
 export_func_3: {
     options = {
+        hoist_funs: true,
         side_effects: true,
+        toplevel: true,
         unused: true,
     }
     input: {
@@ -32,6 +38,8 @@ export_func_3: {
 
 export_default_func_1: {
     options = {
+        hoist_funs: true,
+        toplevel: true,
         unused: true,
     }
     input: {
@@ -42,7 +50,9 @@ export_default_func_1: {
 
 export_default_func_2: {
     options = {
+        hoist_funs: true,
         side_effects: false,
+        toplevel: true,
         unused: true,
     }
     input: {
@@ -53,7 +63,9 @@ export_default_func_2: {
 
 export_default_func_3: {
     options = {
+        hoist_funs: true,
         side_effects: true,
+        toplevel: true,
         unused: true,
     }
     input: {
@@ -61,3 +73,209 @@ export_default_func_3: {
     }
     expect_exact: "export default function(){};"
 }
+
+export_class_1: {
+    options = {
+        hoist_funs: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        export class C {};
+    }
+    expect_exact: "export class C{};"
+}
+
+export_class_2: {
+    options = {
+        hoist_funs: true,
+        side_effects: false,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        export class C {}(1);
+    }
+    expect_exact: "export class C{};1;"
+}
+
+export_class_3: {
+    options = {
+        hoist_funs: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        export class C {}(1);
+    }
+    expect_exact: "export class C{};"
+}
+
+export_default_class_1: {
+    options = {
+        hoist_funs: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        export default class C {};
+    }
+    expect_exact: "export default class{};"
+}
+
+export_default_class_2: {
+    options = {
+        hoist_funs: true,
+        side_effects: false,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        export default class C {}(1);
+    }
+    expect_exact: "export default class{};1;"
+}
+
+export_default_class_3: {
+    options = {
+        hoist_funs: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        export default class C {}(1);
+    }
+    expect_exact: "export default class{};"
+}
+
+export_mangle_1: {
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        export function foo(one, two) {
+            return one - two;
+        };
+    }
+    expect_exact: "export function foo(n,o){return n-o};"
+}
+
+export_mangle_2: {
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        export default function foo(one, two) {
+            return one - two;
+        };
+    }
+    expect_exact: "export default function n(n,r){return n-r};"
+}
+
+export_mangle_3: {
+    options = {
+        collapse_vars: true,
+    }
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        export class C {
+            go(one, two) {
+                var z = one;
+                return one - two + z;
+            }
+        };
+    }
+    expect_exact: "export class C{go(n,r){return n-r+n}};"
+}
+
+export_mangle_4: {
+    options = {
+        collapse_vars: true,
+    }
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        export default class C {
+            go(one, two) {
+                var z = one;
+                return one - two + z;
+            }
+        };
+    }
+    expect_exact: "export default class n{go(n,r){return n-r+n}};"
+}
+
+export_mangle_5: {
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        export default {
+            prop: function(one, two) {
+                return one - two;
+            }
+        };
+    }
+    expect_exact: "export default{prop:function(n,r){return n-r}};"
+}
+
+export_mangle_6: {
+    mangle = {
+        toplevel: true,
+    }
+    input: {
+        var baz = 2;
+        export let foo = 1, bar = baz;
+    }
+    expect_exact: "var a=2;export let foo=1,bar=a;"
+}
+
+export_toplevel_1: {
+    options = {
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function f(){}
+        export function g(){};
+        export default function h(){};
+    }
+    expect: {
+        export function g(){};
+        export default function(){};
+    }
+}
+
+export_toplevel_2: {
+    options = {
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        class A {}
+        export class B {};
+        export default class C {};
+    }
+    expect: {
+        export class B {};
+        export default class {};
+    }
+}
+
+export_default_func_ref: {
+    options = {
+        hoist_funs: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        export default function f(){};
+        f();
+    }
+    expect_exact: "export default function f(){};f();"
+}