Prepare for chaining resynthesizers from AC to SDK AC.

pkg/analyzer/lib/src/context/context.dart

Index: pkg/analyzer/lib/src/context/context.dart
diff --git a/pkg/analyzer/lib/src/context/context.dart b/pkg/analyzer/lib/src/context/context.dart
index 4714557e35a910b1a9a07d958b866ce1cca77006..d9a417ab64cc1cf823c690a3cf992d3cf0533121 100644
--- a/pkg/analyzer/lib/src/context/context.dart
+++ b/pkg/analyzer/lib/src/context/context.dart
@@ -25,6 +25,7 @@ import 'package:analyzer/src/generated/resolver.dart';
 import 'package:analyzer/src/generated/sdk.dart' show DartSdk;
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/utilities_collection.dart';
+import 'package:analyzer/src/summary/resynthesize.dart';
 import 'package:analyzer/src/task/dart.dart';
 import 'package:analyzer/src/task/dart_work_manager.dart';
 import 'package:analyzer/src/task/driver.dart';
@@ -52,6 +53,33 @@ import 'package:html/dom.dart' show Document;
 typedef T PendingFutureComputer<T>(CacheEntry entry);
 
 /**
+ * Helper for [InternalAnalysisContext.aboutToComputeResult].
+ */
+abstract class AboutToComputeResultHelper {
+  final InternalAnalysisContext context;
+
+  /**
+   * The [SummaryResynthesizer] of this context, maybe `null`.
+   */
+  SummaryResynthesizer resynthesizer;
+
+  AboutToComputeResultHelper(this.context);
+
+  /**
+   * This method is invoked by an [InternalAnalysisContext] when the state of
+   * the [result] of the [entry] is [CacheState.INVALID], so it is about to be
+   * computed.
+   *
+   * If the processor knows how to provide the value, it sets the value into
+   * the [entry] with all required dependencies, and returns `true`.
+   *
+   * Otherwise, it returns `false` to indicate that the result should be
+   * computed as usually.
+   */
+  bool compute(CacheEntry entry, ResultDescriptor result);
+}
+
+/**
  * An [AnalysisContext] in which analysis can be performed.
  */
 class AnalysisContextImpl implements InternalAnalysisContext {
@@ -185,6 +213,9 @@ class AnalysisContextImpl implements InternalAnalysisContext {
    */
   List<AnalysisListener> _listeners = new List<AnalysisListener>();
 
+  @override
+  AboutToComputeResultHelper aboutToComputeResultHelper;
+
   /**
    * The most recently incrementally resolved source, or `null` when it was
    * already validated, or the most recent change was not incrementally resolved.
@@ -489,26 +520,21 @@ class AnalysisContextImpl implements InternalAnalysisContext {
   @override
   bool aboutToComputeResult(CacheEntry entry, ResultDescriptor result) {
     return PerformanceStatistics.summary.makeCurrentWhile(() {
-      AnalysisTarget target = entry.target;
-      // TYPE_PROVIDER
-      if (target is AnalysisContextTarget && result == TYPE_PROVIDER) {
-        DartSdk dartSdk = sourceFactory.dartSdk;
-        if (dartSdk != null) {
-          AnalysisContext sdkContext = dartSdk.context;
-          if (!identical(sdkContext, this) &&
-              sdkContext is InternalAnalysisContext) {
-            return sdkContext.aboutToComputeResult(entry, result);
-          }
-        }
+      // Use this helper if it is set.
+      if (aboutToComputeResultHelper != null &&
+          aboutToComputeResultHelper.compute(entry, result)) {
+        return true;
       }
-      // A result for a Source.
-      Source source = target.source;
-      if (source != null) {
-        InternalAnalysisContext context = _cache.getContextFor(source);
-        if (!identical(context, this)) {
-          return context.aboutToComputeResult(entry, result);
+      // Ask the SDK.
+      DartSdk dartSdk = sourceFactory.dartSdk;
+      if (dartSdk != null) {
+        AnalysisContext sdkContext = dartSdk.context;
+        if (!identical(sdkContext, this) &&
+            sdkContext is InternalAnalysisContext) {
+          return sdkContext.aboutToComputeResult(entry, result);
         }
       }
+      // Cannot provide the result.
       return false;
     });
   }