Stub implementation of patch invariants for the patch refactoring.

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
diff --git a/lib/compiler/implementation/compiler.dart b/lib/compiler/implementation/compiler.dart
index 17ed94fb5ebc701034c5630a3868c64cfddf7fc2..2a7a0df55f57b35ee89db14945dd962833e78b2a 100644
--- a/lib/compiler/implementation/compiler.dart
+++ b/lib/compiler/implementation/compiler.dart
@@ -29,7 +29,6 @@ const bool DUMP_INFERRED_TYPES = false;
  */
 const String BUILD_ID = 'build number could not be determined';
 
-
 /**
  * Contains backend-specific data that is used throughout the compilation of
  * one work item.
@@ -39,12 +38,19 @@ class ItemCompilationContext {
 
 class WorkItem {
   final ItemCompilationContext compilationContext;
+  /**
+   * Documentation wanted -- johnniwinther
+   *
+   * Invariant: [element] must be a declaration element.
+   */
   final Element element;
   TreeElements resolutionTree;
   bool allowSpeculativeOptimization = true;
   List<HTypeGuard> guards = const <HTypeGuard>[];
 
-  WorkItem(this.element, this.resolutionTree, this.compilationContext);
+  WorkItem(this.element, this.resolutionTree, this.compilationContext) {
+    assert(invariant(element, element.isDeclaration));
+  }
 
   bool isAnalyzed() => resolutionTree !== null;
 
@@ -608,11 +614,12 @@ class Compiler implements DiagnosticListener {
     assert(phase == PHASE_RECOMPILING);
     while (!world.recompilationCandidates.isEmpty()) {
       WorkItem work = world.recompilationCandidates.next();
-      CodeBuffer oldCode = world.universe.generatedCode[work.element];
-      world.universe.generatedCode.remove(work.element);
-      world.universe.generatedBailoutCode.remove(work.element);
-      withCurrentElement(work.element, () => work.run(this, world));
-      CodeBuffer newCode = world.universe.generatedCode[work.element];
+      Element element = work.element;
+      CodeBuffer oldCode = world.universe.generatedCode[element];
+      world.universe.generatedCode.remove(element);
+      world.universe.generatedBailoutCode.remove(element);
+      withCurrentElement(element, () => work.run(this, world));
+      CodeBuffer newCode = world.universe.generatedCode[element];
       if (REPORT_PASS2_OPTIMIZATIONS && newCode != oldCode) {
         log("Pass 2 optimization:");
         log("Before:\n$oldCode");
@@ -670,6 +677,7 @@ class Compiler implements DiagnosticListener {
   }
 
   TreeElements analyzeElement(Element element) {
+    assert(invariant(element, element.isDeclaration));
     TreeElements elements = enqueuer.resolution.getCachedElements(element);
     if (elements !== null) return elements;
     final int allowed = ElementCategory.VARIABLE | ElementCategory.FUNCTION
@@ -713,6 +721,7 @@ class Compiler implements DiagnosticListener {
                              'Internal error: unresolved element: $element.');
     }
     result = analyzeElement(element);
+    assert(invariant(element, element.isDeclaration));
     enqueuer.resolution.resolvedElements[element] = result;
     return result;
   }
@@ -950,3 +959,24 @@ class SourceSpan {
 
   String toString() => 'SourceSpan($uri, $begin, $end)';
 }
+
+/**
+ * Throws an [InvariantException] if [condition] is [:false:]. [condition] must
+ * be either a [:bool:] or a no-arg function returning a [:bool:].
+ *
+ * Use this method to provide better information for assertion by calling
+ * [invariant] as the argument to an [:assert:] statement:
+ *
+ *     assert(invariant(position, isValid));
+ *
+ * [spannable] must be non-null and will be used to provide positional
+ * information in the generated error message.
+ */
+bool invariant(Spannable spannable, var condition, {String message: null}) {
+  // TODO(johnniwinther): Use [spannable] and [message] to provide better
+  // information on assertion errors.
+  if (condition is Function){
+    condition = condition();
+  }
+  return spannable != null && condition;
+}