fix temps that have the same name to have different Elements

test/codegen/expect/try_catch.js

 var try_catch = dart.defineLibrary(try_catch, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   // Function foo: () → dynamic
   function foo() {
     try {
       throw "hi there";
     } catch (e$) {
       if (dart.is(e$, core.String)) {
@@ skipping 35 matching lines @@
         throw e;
       } else
         throw e;
     }
 
   }
   // Function wub: () → dynamic
   function wub() {
     try {
       throw "on without exception parameter";
-    } catch (e$) {
-      if (dart.is(e$, core.String)) {
+    } catch (e) {

[Jennifer Messerly, 2015/05/11 23:22:01]

the name here should've been a clue ... why `e$` when no other `e` is in this
scope?

+      if (dart.is(e, core.String)) {
       } else
-        throw e$;
+        throw e;
     }
 
   }
   // Function main: () → dynamic
   function main() {
     foo();
     bar();
     baz();
     qux();
     wub();
   }
   // Exports:
   exports.foo = foo;
   exports.bar = bar;
   exports.baz = baz;
   exports.qux = qux;
   exports.wub = wub;
   exports.main = main;
 })(try_catch, core);