Add prefix constraints for deferred imports.

sdk/lib/_internal/compiler/implementation/deferred_load.dart

Index: sdk/lib/_internal/compiler/implementation/deferred_load.dart
diff --git a/sdk/lib/_internal/compiler/implementation/deferred_load.dart b/sdk/lib/_internal/compiler/implementation/deferred_load.dart
index 47ea3a1bc0c4f059188e0f53938d5992644f837a..118521fde79f8647210fccf1762ee23ff484517e 100644
--- a/sdk/lib/_internal/compiler/implementation/deferred_load.dart
+++ b/sdk/lib/_internal/compiler/implementation/deferred_load.dart
@@ -617,13 +617,45 @@ class DeferredLoadTask extends CompilerTask {
     _allDeferredImports[_fakeMainImport] = compiler.mainApp;
     bool deferredUsedFromMain = false;
     var lastDeferred;
+    // When detecting duplicate prefixes of deferred libraries there are 4
+    // cases of duplicate prefixes:
+    // 1.
+    // @DeferredLibrary("a") import "lib.dart" as a;
+    // @DeferredLibrary("b") import "lib2.dart" as a;
+    // 2.
+    // @DeferredLibrary("a") import "lib.dart" as a;
+    // import "lib2.dart" as a;
+    // 3.
+    // import "lib.dart" as a;
+    // @DeferredLibrary("a") import "lib2.dart" as a;
+    // 4.
+    // import "lib.dart" as a;
+    // import "lib2.dart" as a;
+    // We must be able to signal error for case 1, 2, 3, but accept case 4.
+    
+    // The prefixes that have been used by any imports in this library.
+    Setlet<String> usedPrefixes = new Setlet<String>();
+    // The last deferred import we saw with a given prefix (if any).
+    Map<String, Import> prefixDeferredImport = new Map<String, Import>();
     for (LibraryElement library in compiler.libraries.values) {
+      prefixDeferredImport.clear();
+      usedPrefixes.clear();
       // TODO(sigurdm): Make helper getLibraryImportTags when tags is a List
       // instead of a Link.
       for (LibraryTag tag in library.tags) {
         if (tag is! Import) continue;
         Import import = tag;
-        if (_isImportDeferred(import)) {
+        String prefix = import.prefix != null ? import.prefix.toString() : null;

[floitsch, 2014/02/18 12:19:25]

parenthesis around the condition.

[sigurdm, 2014/02/18 13:31:47]

Done.

+        // The last import we saw with the same prefix.
+        Import previousDeferredImport = prefixDeferredImport[prefix];
+        bool isDeferred = _isImportDeferred(import); 
+        if (isDeferred) {
+          if (prefix == null) {
+            compiler.reportError(import,
+                MessageKind.DEFERRED_LIBRARY_WITHOUT_PREFIX);
+          } else {
+            prefixDeferredImport[prefix] = import;
+          }
           splitProgram = true;
           _allDeferredImports[tag] = library.getLibraryFromTag(tag);
           lastDeferred = import.metadata.first;
@@ -631,6 +663,17 @@ class DeferredLoadTask extends CompilerTask {
             deferredUsedFromMain = true;
           }
         }
+        if (prefix != null) {
+          if (previousDeferredImport != null ||
+              (isDeferred && usedPrefixes.contains(prefix))) {
+            Import failingImport = previousDeferredImport != null

[floitsch, 2014/02/18 12:19:25]

parenthesis around condition.

[sigurdm, 2014/02/18 13:31:47]

Done.

+                ? previousDeferredImport
+                : import;
+            compiler.reportError(failingImport,
+                MessageKind.DEFERRED_LIBRARY_DUPLICATE_PREFIX);
+          }
+          usedPrefixes.add(prefix);
+        }
       }
     }
     if (splitProgram && !deferredUsedFromMain) {