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..1f0040aba2deb0df0882fbcd178dc92640658686 100644
--- a/sdk/lib/_internal/compiler/implementation/deferred_load.dart
+++ b/sdk/lib/_internal/compiler/implementation/deferred_load.dart
@@ -617,13 +617,20 @@ class DeferredLoadTask extends CompilerTask {
     _allDeferredImports[_fakeMainImport] = compiler.mainApp;
     bool deferredUsedFromMain = false;
     var lastDeferred;
+    Map<String, List<Import>> prefixImports = new Map<String, List<Import>>();
     for (LibraryElement library in compiler.libraries.values) {
+      prefixImports.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)) {
+          if (import.prefix == null) {
+            compiler.reportError(
+                import,
+                MessageKind.DEFERRED_LIBRARY_WITHOUT_PREFIX);
+          }
           splitProgram = true;
           _allDeferredImports[tag] = library.getLibraryFromTag(tag);
           lastDeferred = import.metadata.first;
@@ -631,6 +638,20 @@ class DeferredLoadTask extends CompilerTask {
             deferredUsedFromMain = true;
           }
         }
+        if (import.prefix != null) {

[floitsch, 2014/02/18 09:45:22]

Add comment explaining why simpler approaches wouldn't work.
Make sure to have test-cases.

(Why do you need a list of imports, ...).

+          prefixImports.putIfAbsent(import.prefix.toString(),
+                                   () => new List<Import>()).add(import);
+        }
+      }
+      for (String prefix in prefixImports.keys) {
+        List<Import> imports = prefixImports[prefix];
+        if (imports.length <= 1) continue;
+        for (Import import in imports) {
+          if (_isImportDeferred(import)) {
+            compiler.reportError(import,
+                MessageKind.DEFERRED_LIBRARY_DUPLICATE_PREFIX);
+          }
+        }
       }
     }
     if (splitProgram && !deferredUsedFromMain) {