Create pkg/front_end/src/base and begin populating with general use classes.

pkg/analyzer/lib/task/model.dart

Index: pkg/analyzer/lib/task/model.dart
diff --git a/pkg/analyzer/lib/task/model.dart b/pkg/analyzer/lib/task/model.dart
index 2a6f6c55df2f66c61ed98822f39966c8b1eaab3e..9d1f6ba1c184b6262fa6d16e2b54f912d3a6804e 100644
--- a/pkg/analyzer/lib/task/model.dart
+++ b/pkg/analyzer/lib/task/model.dart
@@ -14,6 +14,9 @@ import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/utilities_general.dart';
 import 'package:analyzer/src/task/driver.dart';
 import 'package:analyzer/src/task/model.dart';
+import 'package:front_end/src/base/analysis_target.dart';
+
+export 'package:front_end/src/base/analysis_target.dart' show AnalysisTarget;
 
 /**
  * A function that converts the given [key] and [value] into a [TaskInput].
@@ -68,27 +71,6 @@ class AnalysisContextTarget implements AnalysisTarget {
 }
 
 /**
- * An object with which an analysis result can be associated.
- *
- * Clients may implement this class when creating new kinds of targets.
- * Instances of this type are used in hashed data structures, so subtypes are
- * required to correctly implement [==] and [hashCode].
- */
-abstract class AnalysisTarget {
-  /**
-   * If this target is associated with a library, return the source of the
-   * library's defining compilation unit; otherwise return `null`.
-   */
-  Source get librarySource;
-
-  /**
-   * Return the source associated with this target, or `null` if this target is
-   * not associated with a source.
-   */
-  Source get source;
-}
-
-/**
  * An object used to compute one or more analysis results associated with a
  * single target.
  *