Handle invalid deferred prefix declarations.

R=het@google.com

Review URL: https://codereview.chromium.org/2116473002 .

pkg/compiler/lib/src/compiler.dart

@@ -922,7 +922,12 @@ abstract class Compiler implements LibraryLoaderListener {
     library.implementation.forEachLocalMember(enqueueAll);
     library.imports.forEach((ImportElement import) {
       if (import.isDeferred) {
-        world.addToWorkList(import.prefix.loadLibrary);
+        // `import.prefix` and `loadLibrary` may be `null` when the deferred
+        // import has compile-time errors.
+        GetterElement loadLibrary = import.prefix?.loadLibrary;
+        if (loadLibrary != null) {
+          world.addToWorkList(loadLibrary);
+        }
       }
     });
   }

tests/compiler/dart2js/serialization/test_data.dart

@@ -490,6 +490,43 @@ test() {
     'b.dart': '''
 ''',
   }),
+
+  const Test('Deferred without prefix', const {
+    'main.dart': '''
+import 'a.dart';
+
+main() {
+  test();
+}
+''',
+  }, preserializedSourceFiles: const {
+    'a.dart': '''
+import 'b.dart' deferred;
+test() {}
+''',
+    'b.dart': '''
+''',
+  }, expectedErrorCount: 1),
+
+  const Test('Deferred with duplicate prefix', const {
+    'main.dart': '''
+import 'a.dart';
+
+main() {
+  test();
+}
+''',
+  }, preserializedSourceFiles: const {
+    'a.dart': '''
+import 'b.dart' deferred as pre;
+import 'c.dart' deferred as pre;
+test() {}
+''',
+    'b.dart': '''
+''',
+    'c.dart': '''
+''',
+  }, expectedErrorCount: 1),
 ];
 
 class Test {