Fixes #254 - Better stacktrace support

lib/src/codegen/js_codegen.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 2304 matching lines @@
       notNull(node.condition),
       _visit(node.thenExpression),
       _visit(node.elseExpression)
     ]);
   }
 
   @override
   visitThrowExpression(ThrowExpression node) {
     var expr = _visit(node.expression);
     if (node.parent is ExpressionStatement) {
-      return js.statement('throw #;', expr);
+      return js.statement('dart.throw_(#);', expr);

[Jennifer Messerly, 2015/07/13 16:39:17]

commented on the bug, but I have some concerns over never generating `throw`.

Brainstorming how we can get back to it:

* by default, capture stack in expression/error ctor.
* if Error/Exception is immediately thrown, use `throw` statement (or dart.throw
helper for expressions--the old version that just throws)
* otherwise, if we don't know where `e` comes from, fall back to
`dart.captureStackTrace(e)` followed by throw.

And we should probably split stack trace helper APIs in to their own module at
some point.

I think this would fix most of the changed lines in the SDK code, since `throw
new ...` is the common pattern.

[vsm, 2015/07/14 13:54:48]

Done in the case of Error - Exception is only an interface.

     } else {
       return js.call('dart.throw_(#)', expr);
     }
   }
 
   @override
   visitRethrowExpression(RethrowExpression node) {
     if (node.parent is ExpressionStatement) {
-      return js.statement('throw #;', _visit(_catchParameter));
+      return js.statement('dart.rethrow(#);', _visit(_catchParameter));
     } else {
-      return js.call('dart.throw_(#)', _visit(_catchParameter));
+      return js.call('dart.rethrow(#)', _visit(_catchParameter));
     }
   }
 
   @override
   JS.If visitIfStatement(IfStatement node) {
     return new JS.If(notNull(node.condition), _visit(node.thenStatement),
         _visit(node.elseStatement));
   }
 
   @override
@@ skipping 49 matching lines @@
     // This could incorrectly shadow a user's name.
     var savedCatch = _catchParameter;
 
     if (clauses.length == 1 && clauses.single.exceptionParameter != null) {
       // Special case for a single catch.
       _catchParameter = clauses.single.exceptionParameter;
     } else {
       _catchParameter = _createTemporary('e', types.dynamicType);
     }
 
-    JS.Statement catchBody = js.statement('throw #;', _visit(_catchParameter));
+    JS.Statement catchBody = js.statement('dart.rethrow(#);', _visit(_catchParameter));
     for (var clause in clauses.reversed) {
       catchBody = _catchClauseGuard(clause, catchBody);
     }
 
     var catchVarDecl = _visit(_catchParameter);
     _catchParameter = savedCatch;
     return new JS.Catch(catchVarDecl, new JS.Block([catchBody]));
   }
 
   JS.Statement _catchClauseGuard(CatchClause clause, JS.Statement otherwise) {
@@ skipping 406 matching lines @@
 
 class _JsThisFinder extends JS.BaseVisitor {
   bool found = false;
   visitThis(JS.This node) {
     found = true;
   }
   visitNode(JS.Node node) {
     if (!found) super.visitNode(node);
   }
 }