compress undefined property names (#2811)

- enforce property names as string
- handle `void 0` as `undefined` in `hoist_props` & `reduce_vars`

test/compress/hoist_props.js

@@ -664,3 +664,25 @@ issue_2519: {
     }
     expect_stdout: "5.5"
 }
+
+undefined_key: {
+    options = {
+        evaluate: true,
+        hoist_props: true,
+        join_vars: true,
+        passes: 4,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a, o = {};
+        o[a] = 1;
+        o.b = 2;
+        console.log(o[a] + o.b);
+    }
+    expect: {
+        console.log(3);
+    }
+    expect_stdout: "3"
+}

test/compress/issue-1770.js

@@ -84,12 +84,12 @@ numeric_literal: {
         '    0: 0,',
         '    "-0": 1,',
         '    42: 2,',
-        '    "42": 3,',
+        '    42: 3,',
         '    37: 4,',
         '    o: 5,',
         '    1e42: 6,',
         '    b: 7,',
-        '    "1e+42": 8',
+        '    1e42: 8',
         '};',
         '',
         'console.log(obj[-0], obj[-""], obj["-0"]);',