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..1b6dcc65f39dd49a6be7ec0acc29c333afda4678 100644
--- a/sdk/lib/_internal/compiler/implementation/deferred_load.dart
+++ b/sdk/lib/_internal/compiler/implementation/deferred_load.dart
@@ -618,12 +618,18 @@ class DeferredLoadTask extends CompilerTask {
     bool deferredUsedFromMain = false;
     var lastDeferred;
     for (LibraryElement library in compiler.libraries.values) {
+      Map<String, List<Import>> prefixImports = {};

[floitsch, 2014/02/17 14:39:26]

Move outside the loop, and just clear it at every iteration.

[sigurdm, 2014/02/18 08:49:53]

Done.

       // 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 +637,20 @@ class DeferredLoadTask extends CompilerTask {
             deferredUsedFromMain = true;
           }
         }
+        if (import.prefix != null) {
+          prefixImports.putIfAbsent(import.prefix.toString(),
+                                   () => new List<Import>()).add(import);

[floitsch, 2014/02/17 14:39:26]

Instead of allocating a new closure/list every time, can't we just report the
error here?

Since you don't use the original import in the error-message, you can even
switch to a Setlet for the used prefixes.

Something along those lines:

usedPrefixes = new Setlet<String>();

if (usedPrefixes.contains(import.prefix.toString()) {
  if (!_isImportDeferred(import)) {
    // Find deferred import with same prefix and assign it to "import".
  }
  compiler.reportError(import, MessageKind.DEFERRED_LIBRARY_DUPLICATE_PREFIX);
} else {
  usedPrefixes.add(import.prefix.toString());
}

[sigurdm, 2014/02/18 08:49:53]

I wanted to do something like that first - but it seemed less simple to make
that work.

We still want to allow collisions for non-deferred imports:

import 'a.dart' as a;
import 'b.dart' as a;

So we would have to remember for each prefix both if we saw "any imports" and if
we saw "any deferred imports".

+        }
+      }
+      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) {