Fixes #254 - Better stacktrace support

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 4426755e2b6e30fb7962793d18e1c5b3c8606f7c..4dc3022f25bd04a331ae4057c5db601f80405bf8 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -2322,7 +2322,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor {
   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);
     }
@@ -2331,9 +2331,9 @@ class JSCodegenVisitor extends GeneralizingAstVisitor {
   @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));
     }
   }
 
@@ -2403,7 +2403,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor {
       _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);
     }