Add initial support for parts to the incremental kernel generator.

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

Index: pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
diff --git a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
index 8ea6d18cbb4a5e46011fa5e8bff9407a445b9418..4221ba8f2f2b21bc71a14ec2b54bff25c22deb5d 100644
--- a/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
+++ b/pkg/front_end/lib/src/incremental_kernel_generator_impl.dart
@@ -116,9 +116,13 @@ class _AnalysisContextProxy implements AnalysisContext {
   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];
+    }
   }
 }