Add Member.documentationComment and use it to resynthesize documentation from Kernel.

pkg/front_end/lib/src/fasta/kernel/kernel_target.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
index eb8fb960a849c9c99619fcf836d800884dcfe229..d52ad3eb58fef04da0d0a0c8f47b37cec288aef7 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -331,8 +331,19 @@ class KernelTarget extends TargetImplementation {
       // method. Similarly considerations apply to separate compilation. It
       // could also make sense to add a way to mark .dill files as having
       // compile-time errors.
-      KernelProcedureBuilder mainBuilder = new KernelProcedureBuilder(null, 0,
-          null, "main", null, null, ProcedureKind.Method, library, -1, -1, -1);
+      KernelProcedureBuilder mainBuilder = new KernelProcedureBuilder(
+          null,
+          null,
+          0,
+          null,
+          "main",
+          null,
+          null,
+          ProcedureKind.Method,
+          library,
+          -1,
+          -1,
+          -1);
       library.addBuilder(mainBuilder.name, mainBuilder, -1);
       mainBuilder.body = new Block(new List<Statement>.from(errors.map(
           (LocatedMessage message) => new ExpressionStatement(new Throw(