dart2js: add support for configuration-specific imports.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 6ebb7a56ba782611176b0a4fd6e79451fc10195c..6ebee8f5e242758e8f845fbcc94ed89ff784d434 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -520,6 +520,7 @@ abstract class Compiler extends DiagnosticListener {
             this.dumpInfo: false,
             this.showPackageWarnings: false,
             bool useStartupEmitter: false,
+            bool enableConditionalDirectives: false,
             this.useContentSecurityPolicy: false,
             this.suppressWarnings: false,
             this.fatalWarnings: false,
@@ -582,9 +583,12 @@ abstract class Compiler extends DiagnosticListener {
       libraryLoader = new LibraryLoaderTask(this),
       serialization = new SerializationTask(this),
       scanner = new ScannerTask(this),
-      dietParser = new DietParserTask(this),
-      parser = new ParserTask(this),
-      patchParser = new PatchParserTask(this),
+      dietParser = new DietParserTask(
+          this, enableConditionalDirectives: enableConditionalDirectives),
+      parser = new ParserTask(
+          this, enableConditionalDirectives: enableConditionalDirectives),
+      patchParser = new PatchParserTask(
+          this, enableConditionalDirectives: enableConditionalDirectives),
       resolver = new ResolverTask(this, backend.constantCompilerTask),
       closureToClassMapper = new closureMapping.ClosureTask(this),
       checker = new TypeCheckerTask(this),