Properly pass environment variables from pub to dart2js.

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

Index: sdk/lib/_internal/pub/lib/src/dart.dart
diff --git a/sdk/lib/_internal/pub/lib/src/dart.dart b/sdk/lib/_internal/pub/lib/src/dart.dart
index 89845af29d46a1dbc12343a0795196d328d3140d..e160572d3ad35a10d9ee3b67405e3fad93c4d42d 100644
--- a/sdk/lib/_internal/pub/lib/src/dart.dart
+++ b/sdk/lib/_internal/pub/lib/src/dart.dart
@@ -72,12 +72,7 @@ Future compile(String entrypoint, CompilerProvider provider, {
     if (terse) options.add('--terse');
     if (toDart) options.add('--output-type=dart');
 
-    if (environment != null) {
-      environment.forEach((name, value) {
-        options.add('-D$name=$value');
-      });
-    }
-
+    if (environment == null) environment = {};
     if (commandLineOptions != null) options.addAll(commandLineOptions);
 
     if (packageRoot == null) {
@@ -91,7 +86,8 @@ Future compile(String entrypoint, CompilerProvider provider, {
         provider.provideInput,
         provider.handleDiagnostic,
         options,
-        provider.provideOutput));
+        provider.provideOutput,
+        environment));
   });
 }