"Implement" #resource tag and various restrictions on library definitions.

dart/frog/tests/leg/src/mock_compiler.dart

Index: dart/frog/tests/leg/src/mock_compiler.dart
diff --git a/dart/frog/tests/leg/src/mock_compiler.dart b/dart/frog/tests/leg/src/mock_compiler.dart
index 073be0184cd2333d285fc3287def03c88ff5000c..20e1c8f9f37dfd122016fb5d79f65f1a7d3e8766 100644
--- a/dart/frog/tests/leg/src/mock_compiler.dart
+++ b/dart/frog/tests/leg/src/mock_compiler.dart
@@ -75,6 +75,10 @@ class MockCompiler extends Compiler {
   }
 
   void reportError(Node node, var message) {
+    if (message is String && message.startsWith("no #library tag found in")) {
+      // TODO(ahe): Fix the MockCompiler to not have this problem.
+      return;
+    }
     errors.add(new WarningMessage(node, message.message));
   }