Enable conditional directives by default.

pkg/compiler/lib/src/parser/parser.dart

Index: pkg/compiler/lib/src/parser/parser.dart
diff --git a/pkg/compiler/lib/src/parser/parser.dart b/pkg/compiler/lib/src/parser/parser.dart
index 42e432becd9afb93159aeec24e8c832ec9477ad6..9122a1c8c9db76997fc1f8f51a2fecf69157b42d 100644
--- a/pkg/compiler/lib/src/parser/parser.dart
+++ b/pkg/compiler/lib/src/parser/parser.dart
@@ -98,12 +98,10 @@ class FormalParameterType {
 class Parser {
   final Listener listener;
   bool mayParseFunctionExpressions = true;
-  final bool enableConditionalDirectives;
   bool asyncAwaitKeywordsEnabled;
 
   Parser(this.listener,
-      {this.enableConditionalDirectives: false,
-       this.asyncAwaitKeywordsEnabled: false});
+      {this.asyncAwaitKeywordsEnabled: false});
 
   Token parseUnit(Token token) {
     listener.beginCompilationUnit(token);
@@ -180,11 +178,9 @@ class Parser {
   Token parseConditionalUris(Token token) {
     listener.beginConditionalUris(token);
     int count = 0;
-    if (enableConditionalDirectives) {
-      while (optional('if', token)) {
-        count++;
-        token = parseConditionalUri(token);
-      }
+    while (optional('if', token)) {
+      count++;
+      token = parseConditionalUri(token);
     }
     listener.endConditionalUris(count);
     return token;