Add initial support for parts to the incremental kernel generator.

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
@@ skipping 98 matching lines @@
         .elementDeclaredByCompilationUnit(
             _resolvedLibraries[source.uri].definingCompilationUnit)
         .library;
   }
 
   noSuchMethod(Invocation invocation) => unimplemented();
 
   CompilationUnit resolveCompilationUnit(
       Source unitSource, LibraryElement library) {
     assert(_resolvedLibraries.containsKey(library.source.uri));
-    // TODO(paulberry): support parts.
-    assert(unitSource == library.source);
-    return _resolvedLibraries[library.source.uri].definingCompilationUnit;
+    var resolvedLibrary = _resolvedLibraries[library.source.uri];
+    if (unitSource == library.source) {
+      return resolvedLibrary.definingCompilationUnit;
+    } else {
+      assert(resolvedLibrary.partUnits.containsKey(unitSource.uri));
+      return resolvedLibrary.partUnits[unitSource.uri];
+    }
   }
 }
 
 class _AnalysisOptionsProxy implements AnalysisOptions {
   final bool strongMode;
 
   _AnalysisOptionsProxy(this.strongMode);
 
   noSuchMethod(Invocation invocation) => unimplemented();
 }
 
 class _SourceFactoryProxy implements SourceFactory {
   Source forUri2(Uri absoluteUri) => new _SourceProxy(absoluteUri);
 
   noSuchMethod(Invocation invocation) => unimplemented();
 }
 
 class _SourceProxy extends BasicSource {
   _SourceProxy(Uri uri) : super(uri);
 
   noSuchMethod(Invocation invocation) => unimplemented();
 }