Fix various semantic nits that Fasta will detect.

sdk/lib/_internal/js_runtime/lib/js_number.dart

Index: sdk/lib/_internal/js_runtime/lib/js_number.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_number.dart b/sdk/lib/_internal/js_runtime/lib/js_number.dart
index 9d2351f129debf31a4aa1f4bab8cea832b4f51f5..e2a3be667154bd75758df4c95582b3cbe38ca09e 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_number.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_number.dart
@@ -220,7 +220,7 @@ class JSNumber extends Interceptor implements num {
       // Then we don't know how to handle it at all.
       throw new UnsupportedError("Unexpected toString result: $result");
     }
-    String result = JS('String', '#', match[1]);
+    result = JS('String', '#', match[1]);

[ahe, 2017/05/23 14:44:27]

Already used above in this scope.

     int exponent = JS("int", "+#", match[3]);
     if (match[2] != null) {
       result = JS('String', '# + #', result, match[2]);