feat(transiler/dart): re-export imported vars

```
import {Foo} from ‘./foo’;
var localVar = true;
export {Foo, localVar};

===>

import ‘./foo’ show Foo;
export ‘./foo’ show Foo;
var localVar = true;
```

Closes #41

tools/transpiler/spec/baz.js

@@ -0,0 +1 @@
+export var Baz = 'BAZ';

tools/transpiler/spec/imports_spec.js

@@ -9,8 +9,14 @@ import * as exportModule from './export';
 
 import {Type} from 'facade/lang';
 
+import {Baz} from './reexport';
+
 export function main() {
   describe('imports', function() {
+    it('should re-export imported vars', function() {
+      expect(Baz).toBe('BAZ');
+    });
+
     it('should work', function() {
       expect(Foo).toBe('FOO');
       expect(Bar).toBe('BAR');

tools/transpiler/spec/reexport.js

@@ -0,0 +1,10 @@
+import {Baz} from './baz';
+import {Bar1} from './bar';
+
+var localVar = true;
+
+export {Baz, localVar, Bar1};
+
+// Will become:
+// export {Baz} from './baz';
+// export {Bar1} from './bar';