First class block scope

- Make let, const, and class symbols be declared in a block scope.
- Piggy back on existing catch symbol implementation to get block-aware mangling working
- Make sure unused block-scoped declarations can be dropped
- Don't eliminate a block if it has a block-scoped declaration
- Remove silly empty anonymous blocks left over from drop_unused
- AST_Toplevel now gets to call drop_unused too, since block-scoped variables aren't global!
- Don't consider block declarations global

test/compress/block-scope.js

@@ -31,3 +31,103 @@ do_not_hoist_let: {
     }
 }
 
+do_not_remove_anon_blocks_if_they_have_decls: {
+    input: {
+        function x() {
+            {
+                let x;
+            }
+            {
+                var x;
+            }
+            {
+                const y;
+                class Zee {};
+            }
+        }
+        {
+            let y;
+        }
+        {
+            var y;
+        }
+    }
+    expect: {
+        function x(){
+            {
+                let x
+            }
+            var x;
+            {
+                const y;
+                class Zee {}
+            }
+        }
+        {
+            let y
+        }
+        var y;
+    }
+}
+
+remove_unused_in_global_block: {
+    options = {
+        unused: true,
+    }
+    input: {
+        {
+            let x;
+            const y;
+            class Zee {};
+            var w;
+        }
+        let ex;
+        const why;
+        class Zed {};
+        var wut;
+        console.log(x, y, Zee);
+    }
+    expect: {
+        var w;
+        var wut;
+        console.log(x, y, Zee);
+    }
+}
+
+regression_block_scope_resolves: {
+    mangle = { };
+    options = {
+        dead_code: false
+    };
+    input: {
+        (function () {
+            if(1) {
+                let x;
+                const y;
+                class Zee {};
+            }
+            if(1) {
+                let ex;
+                const why;
+                class Zi {};
+            }
+            console.log(x, y, Zee, ex, why, Zi);
+        }());
+    }
+    expect: {
+        (function () {
+            if (1) {
+                let a;
+                const b;
+                class c {};
+            }
+            if (1) {
+                let a;
+                const b;
+                class c {};
+            }
+            console.log(x, y, Zee, ex, why, Zi);
+        }());
+    }
+}
+