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/ast.js

@@ -282,9 +282,10 @@ var AST_With = DEFNODE("With", "expression", {
 
 /* -----[ scope and functions ]----- */
 
-var AST_Scope = DEFNODE("Scope", "directives variables functions uses_with uses_eval parent_scope enclosed cname", {
+var AST_Scope = DEFNODE("Scope", "is_block_scope directives variables functions uses_with uses_eval parent_scope enclosed cname", {
     $documentation: "Base class for all statements introducing a lexical scope",
     $propdoc: {
+        is_block_scope: "[boolean] identifies a block scope",
         directives: "[string*/S] an array of directives declared in this scope",
         variables: "[Object/S] a map of name -> SymbolDef for all variables/functions defined in this scope",
         functions: "[Object/S] like `variables`, but only lists function declarations",
@@ -1077,9 +1078,17 @@ var AST_SymbolVar = DEFNODE("SymbolVar", null, {
     $documentation: "Symbol defining a variable",
 }, AST_SymbolDeclaration);
 
+var AST_SymbolBlockDeclaration = DEFNODE("SymbolBlockDeclaration", null, {
+    $documentation: "Base class for block-scoped declaration symbols"
+}, AST_SymbolDeclaration);
+
 var AST_SymbolConst = DEFNODE("SymbolConst", null, {
     $documentation: "A constant declaration"
-}, AST_SymbolDeclaration);
+}, AST_SymbolBlockDeclaration);
+
+var AST_SymbolLet = DEFNODE("SymbolLet", null, {
+    $documentation: "A block-scoped `let` declaration"
+}, AST_SymbolBlockDeclaration);
 
 var AST_SymbolFunarg = DEFNODE("SymbolFunarg", null, {
     $documentation: "Symbol naming a function argument",
@@ -1099,7 +1108,7 @@ var AST_SymbolLambda = DEFNODE("SymbolLambda", null, {
 
 var AST_SymbolDefClass = DEFNODE("SymbolDefClass", null, {
     $documentation: "Symbol naming a class's name in a class declaration. Lexically scoped to its containing scope, and accessible within the class."
-}, AST_SymbolDeclaration);
+}, AST_SymbolBlockDeclaration);
 
 var AST_SymbolClass = DEFNODE("SymbolClass", null, {
     $documentation: "Symbol naming a class's name. Lexically scoped to the class."
@@ -1107,11 +1116,11 @@ var AST_SymbolClass = DEFNODE("SymbolClass", null, {
 
 var AST_SymbolCatch = DEFNODE("SymbolCatch", null, {
     $documentation: "Symbol naming the exception in catch",
-}, AST_SymbolDeclaration);
+}, AST_SymbolBlockDeclaration);
 
 var AST_SymbolImport = DEFNODE("SymbolImport", null, {
     $documentation: "Symbol refering to an imported name",
-}, AST_SymbolDeclaration);
+}, AST_SymbolBlockDeclaration);
 
 var AST_SymbolImportForeign = DEFNODE("SymbolImportForeign", null, {
     $documentation: "A symbol imported from a module, but it is defined in the other module, and its real name is irrelevant for this module's purposes",