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

lib/parse.js

@@ -1214,11 +1214,14 @@ function parse($TEXT, options) {
         });
     };
 
-    function vardefs(no_in, in_const) {
+    function vardefs(no_in, kind) {
         var a = [];
         var def;
         for (;;) {
-            var sym_type = in_const ? AST_SymbolConst : AST_SymbolVar;
+            var sym_type =
+                kind === "var" ? AST_SymbolVar :
+                kind === "const" ? AST_SymbolConst :
+                kind === "let" ? AST_SymbolLet : null;
             if (is("punc", "{") || is("punc", "[")) {
                 def = new AST_VarDef({
                     start: S.token,
@@ -1287,7 +1290,7 @@ function parse($TEXT, options) {
     var var_ = function(no_in) {
         return new AST_Var({
             start       : prev(),
-            definitions : vardefs(no_in, false),
+            definitions : vardefs(no_in, "var"),
             end         : prev()
         });
     };
@@ -1295,7 +1298,7 @@ function parse($TEXT, options) {
     var let_ = function(no_in) {
         return new AST_Let({
             start       : prev(),
-            definitions : vardefs(no_in, false),
+            definitions : vardefs(no_in, "let"),
             end         : prev()
         });
     };
@@ -1303,7 +1306,7 @@ function parse($TEXT, options) {
     var const_ = function() {
         return new AST_Const({
             start       : prev(),
-            definitions : vardefs(false, true),
+            definitions : vardefs(false, "const"),
             end         : prev()
         });
     };