Add support for not including platform.js in build output

pkg/polymer/lib/src/build/common.dart

Index: pkg/polymer/lib/src/build/common.dart
diff --git a/pkg/polymer/lib/src/build/common.dart b/pkg/polymer/lib/src/build/common.dart
index d75c58561b114e1506af4b158de2aede48e3c064..8ef384c70b70341c65f3dd73383e98c659c250bb 100644
--- a/pkg/polymer/lib/src/build/common.dart
+++ b/pkg/polymer/lib/src/build/common.dart
@@ -93,10 +93,14 @@ class TransformOptions {
   // reachable (entry point+imported) html if deploying. See dartbug.com/17199.
   final bool lint;
 
+  /// This will automatically inject `platform.js` from the `web_components`
+  /// package in all entry points, if it is not already included.
+  final bool injectPlatformJs;
+
   TransformOptions({entryPoints, this.inlineStylesheets,
       this.contentSecurityPolicy: false, this.directlyIncludeJS: true,
       this.releaseMode: true, this.lint: true,
-      this.injectBuildLogsInOutput: false})
+      this.injectBuildLogsInOutput: false, this.injectPlatformJs: true})
       : entryPoints = entryPoints == null ? null
           : entryPoints.map(systemToAssetPath).toList();