Fixes in codegen and test script

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 1c70c19e778f183bf0a2e1a6ead9be4107a7bd07..2cc40c1fca4429318a19334abc865f37f837fee5 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -1537,7 +1537,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     if (node.catchKeyword != null) {
       var name = node.exceptionParameter;
       if (name != null && name.name != _catchParameter) {
-        body.add(js.statement('let # = #;', [_visit(name), _catchParameter]));

[Jennifer Messerly, 2015/03/11 20:28:54]

hmmm. This is not going to have source location associated with the variable.
Maybe not a big deal. (maybe we should put it on the assignment statement
anyway)

Curious, what did this generate as? I'm worried if our visitSimpleIdentifier
logic isn't catching all of these cases.  If it's wrong here it would be wrong
in other cases (right?)... I think it started too simple -- Analyzer's visitors
tend to do more significant series of checks. Probably we should visit the
Element and make sure all of the cases are handled.

(another thing we should probably be using more is inSetterContext)

[Siggi Cherem (dart-lang), 2015/03/11 23:27:22]

Oh, I see.
Here the problem was that visitSimpleIdentifier is assuming that identifiers are
expressions, so it was returning a VariableUse. The let statement expects a
VariableDeclaration or a name, so for example, here is the error you'd get from
js_ast with the regression test (`try_catch.dart`):


Interpolated value #0 is not a VariableDeclaration or String: VariableUse(e)
#0      InstantiatorGeneratorVisitor.error
(package:dev_compiler/src/js/template.dart:154:5)
#1     
InstantiatorGeneratorVisitor.visitInterpolatedVariableDeclaration.<anonymous
closure> (package:dev_compiler/src/js/template.dart:295:19)
#2      InstantiatorGeneratorVisitor.visitVariableInitialization.<anonymous
closure> (package:dev_compiler/src/js/template.dart:584:26)
#3      InstantiatorGeneratorVisitor.visitVariableDeclarationList.<anonymous
closure> (package:dev_compiler/src/js/template.dart:560:34)
#4      InstantiatorGeneratorVisitor.visitExpressionStatement.<anonymous
closure> (package:dev_compiler/src/js/template.dart:361:29)
#5      Template.instantiate (package:dev_compiler/src/js/template.dart:104:26)
#6      JsBuilder.statement (package:dev_compiler/src/js/builder.dart:224:32)
#7      JSCodegenVisitor.visitCatchClause
(package:dev_compiler/src/codegen/js_codegen.dart:1540:30)
#8      CatchClause.accept (package:analyzer/src/generated/ast.dart:4078:57)
#9      JSCodegenVisitor._visit
(package:dev_compiler/src/codegen/js_codegen.dart:1730:29)
#10     JSCodegenVisitor._visitList
(package:dev_compiler/src/codegen/js_codegen.dart:1743:46)
#11     JSCodegenVisitor._visitCatch
(package:dev_compiler/src/codegen/js_codegen.dart:1518:44)
#12     JSCodegenVisitor.visitTryStatement
(package:dev_compiler/src/codegen/js_codegen.dart:1497:53)
...


It seemed strange to change the behavior on visitSimpelIdentifier, so instead I
went ahead and created the node with the sourcespan info here.

[Jennifer Messerly, 2015/03/11 23:48:00]

ouch, yeah, it seems like my Template.visitInterpolatedVariableDeclaration is
not general enough to handle that case :|

I'd probably put the blame here on the JS AST structure not quite being what we
want. But yeah, your new code is fine to do, and it's the same workaround we do
elsewhere.

+        body.add(js.statement('let # = #;', [name.name, _catchParameter]));
         _catchParameter = name.name;
       }
       if (node.stackTraceParameter != null) {