Support defining environment constants for dart2js via the command-line.

sdk/lib/_internal/pub/lib/src/command/serve.dart

Index: sdk/lib/_internal/pub/lib/src/command/serve.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/serve.dart b/sdk/lib/_internal/pub/lib/src/command/serve.dart
index 32074fc90fbd20bb3479b8148d5a21c5cd091b42..c5b47469379f7a510c2f5288c34d6e165c6ae0c5 100644
--- a/sdk/lib/_internal/pub/lib/src/command/serve.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/serve.dart
@@ -59,6 +59,9 @@ class ServeCommand extends BarbackCommand {
   final _completer = new Completer();
 
   ServeCommand() {
+    argParser.addOption("define", abbr: "D",
+        help: "Defines an environment constant for dart2js.",
+        allowMultiple: true, splitCommas: false);
     argParser.addOption('hostname', defaultsTo: 'localhost',
         help: 'The hostname to listen on.');
     argParser.addOption('port', defaultsTo: '8080',
@@ -88,9 +91,13 @@ class ServeCommand extends BarbackCommand {
     var watcherType = argResults['force-poll'] ?
         WatcherType.POLLING : WatcherType.AUTO;
 
+    var environmentConstants = new Map.fromIterable(argResults["define"],
+        key: (pair) => pair.split("=").first,
+        value: (pair) => pair.split("=").last);
+
     var environment = await AssetEnvironment.create(entrypoint, mode,
         watcherType: watcherType, hostname: hostname, basePort: port,
-        useDart2JS: useDart2JS);
+        useDart2JS: useDart2JS, environmentConstants: environmentConstants);
     var directoryLength = sourceDirectories.map((dir) => dir.length)
         .reduce(math.max);