more care around generated names, fixes #60 #82 and #97

test/codegen/try_catch.dart

-main() {
+// 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.
+
+foo() {
   try {
     throw "hi there";
   } on String catch (e, t) {
   } catch (e, t) {
     rethrow;
   }
 }
-
+bar() {
+  try {
+    throw "hi there";
+  } catch (e, t) {
+  } on String catch (e, t) {
+    // unreachable
+    rethrow;
+  }
+}
+main() {
+  foo();
+  bar();
+}