Check for duplicate library names

lib/runtime/dart/_foreign_helper.js

-var _foreign_helper = dart.defineLibrary(_foreign_helper, {});
-var core = dart.import(core);
+dart.defineLibrary('_foreign_helper');
+dart.import('core');
 (function(exports, core) {
   'use strict';
   function JS(typeDescription, codeTemplate, arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9, arg10, arg11) {
     if (arg0 === void 0)
       arg0 = null;
     if (arg1 === void 0)
       arg1 = null;
     if (arg2 === void 0)
       arg2 = null;
     if (arg3 === void 0)
@@ skipping 136 matching lines @@
   exports.JS_FUNCTION_TYPE_RETURN_TYPE_TAG = JS_FUNCTION_TYPE_RETURN_TYPE_TAG;
   exports.JS_FUNCTION_TYPE_REQUIRED_PARAMETERS_TAG = JS_FUNCTION_TYPE_REQUIRED_PARAMETERS_TAG;
   exports.JS_FUNCTION_TYPE_OPTIONAL_PARAMETERS_TAG = JS_FUNCTION_TYPE_OPTIONAL_PARAMETERS_TAG;
   exports.JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG = JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG;
   exports.JS_GET_NAME = JS_GET_NAME;
   exports.JS_EMBEDDED_GLOBAL = JS_EMBEDDED_GLOBAL;
   exports.JS_GET_FLAG = JS_GET_FLAG;
   exports.JS_EFFECT = JS_EFFECT;
   exports.JS_CONST = JS_CONST;
   exports.JS_STRING_CONCAT = JS_STRING_CONCAT;
 })(_foreign_helper, core);

[Jennifer Messerly, 2015/06/05 16:22:27]

fyi, some tools, like Closure, don't like a variable that appears to be
uninitialized. Similarly, I don't think it will work in node.js. That's the
benefit of declaring it. e.g. see this patch:
https://github.com/webcomponents/webcomponentsjs/pull/327
(those polyfills are browser only, so safe to use `window` ... I don't think
`window` would work for us)

[vsm, 2015/06/05 17:01:03]

I'd been thinking about keeping this explicit at decl.  E.g.,:

var _foreign_helper = dart.defineLibrary('_foreign_helper');

WDYT?