Add version validation for LookupMap, also add unittest directly in LookupMap (take 2)

pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart

Index: pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart b/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
index eeea65063e702941c21001c6340acc17cc68dc3c..be859799a70c1e443c9cbcfa82abd429a808dd7d 100644
--- a/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/lookup_map_analysis.dart
@@ -7,6 +7,7 @@ library compiler.src.js_backend.lookup_map_analysis;
 
 import '../common/registry.dart' show Registry;
 import '../compiler.dart' show Compiler;
+import '../diagnostics/messages.dart' show MessageKind;
 import '../constants/values.dart' show
      ConstantValue,
      ConstructedConstantValue,
@@ -14,8 +15,15 @@ import '../constants/values.dart' show
      NullConstantValue,
      TypeConstantValue;
 import '../dart_types.dart' show DartType;
-import '../elements/elements.dart' show Elements, Element, ClassElement,
-     FieldElement, FunctionElement, FunctionSignature;
+import '../elements/elements.dart' show
+    ClassElement,
+    Element,
+    Elements,
+    FieldElement,
+    FunctionElement,
+    FunctionSignature,
+    LibraryElement,
+    VariableElement;
 import '../enqueue.dart' show Enqueuer;
 import 'js_backend.dart' show JavaScriptBackend;
 import '../dart_types.dart' show DynamicType, InterfaceType;
@@ -63,6 +71,13 @@ class LookupMapAnalysis {
   /// discover that a key in a map is potentially used.
   final JavaScriptBackend backend;
 
+  /// The resolved [VariableElement] associated with the top-level `_version`.
+  VariableElement lookupMapVersionVariable;
+
+  /// The resolved [LibraryElement] associated with
+  /// `package:lookup_map/lookup_map.dart`.
+  LibraryElement lookupMapLibrary;
+
   /// The resolved [ClassElement] associated with `LookupMap`.
   ClassElement typeLookupMapClass;
 
@@ -101,9 +116,7 @@ class LookupMapAnalysis {
   final _pending = <ConstantValue, List<_LookupMapInfo>>{};
 
   /// Whether the backend is currently processing the codegen queue.
-  // TODO(sigmund): is there a better way to do this. Do we need to plumb the
-  // enqueuer on each callback?
-  bool get _inCodegen => backend.compiler.phase == Compiler.PHASE_COMPILING;
+  bool _inCodegen = false;
 
   LookupMapAnalysis(this.backend);
 
@@ -114,14 +127,53 @@ class LookupMapAnalysis {
     return typeLookupMapClass != null;
   }
 
-  /// Initializes this analysis by providing the resolver information of
-  /// `LookupMap`.
-  void initRuntimeClass(ClassElement cls) {
+  /// Initializes this analysis by providing the resolved library. This is
+  /// invoked during resolution when the `lookup_map` library is discovered.
+  void init(LibraryElement library) {
+    lookupMapLibrary = library;
+    // We will enable the lookupMapAnalysis as long as we get a known version of
+    // the lookup_map package. We otherwise produce a warning.
+    lookupMapVersionVariable = library.implementation.findLocal('_version');
+    if (lookupMapVersionVariable == null) {
+      backend.compiler.reportInfo(library,
+          MessageKind.UNRECOGNIZED_VERSION_OF_LOOKUP_MAP);
+    } else {
+      backend.compiler.enqueuer.resolution.addToWorkList(
+          lookupMapVersionVariable);
+    }
+  }
+
+  /// Checks if the version of lookup_map is valid, and if so, enable this
+  /// analysis during codegen.
+  void onCodegenStart() {
+    _inCodegen = true;
+    if (lookupMapVersionVariable == null) return;
+
+    // At this point, the lookupMapVersionVariable should be resolved and it's
+    // constant value should be available.
+    ConstantValue value =
+        backend.constants.getConstantValueForVariable(lookupMapVersionVariable);
+    if (value == null) {
+      backend.compiler.reportInfo(lookupMapVersionVariable,
+          MessageKind.UNRECOGNIZED_VERSION_OF_LOOKUP_MAP);
+      return;
+    }
+
+    // TODO(sigmund): add proper version resolution using the pub_semver package
+    // when we introduce the next version.
+    String version = value.primitiveValue.slowToString();
+    if (version != '0.0.1') {
+      backend.compiler.reportInfo(lookupMapVersionVariable,
+          MessageKind.UNRECOGNIZED_VERSION_OF_LOOKUP_MAP);
+      return;
+    }
+
+    ClassElement cls = lookupMapLibrary.findLocal('LookupMap');
     cls.computeType(backend.compiler);
     entriesField = cls.lookupMember('_entries');
     keyField = cls.lookupMember('_key');
     valueField = cls.lookupMember('_value');
-    // TODO(sigmund): Maybe inline nested maps make the output code smaller?
+    // TODO(sigmund): Maybe inline nested maps to make the output code smaller?
     typeLookupMapClass = cls;
   }