Implement the new '?.' operator in analyzer.

pkg/analyzer/lib/src/generated/ast.dart

Index: pkg/analyzer/lib/src/generated/ast.dart
diff --git a/pkg/analyzer/lib/src/generated/ast.dart b/pkg/analyzer/lib/src/generated/ast.dart
index c2e2ab20da9135312af011fa4a2a33643f97a5bf..40fd83a45bf4f7ba69491c9f19b7266d3fa30e91 100644
--- a/pkg/analyzer/lib/src/generated/ast.dart
+++ b/pkg/analyzer/lib/src/generated/ast.dart
@@ -18156,7 +18156,10 @@ class ToSourceVisitor implements AstVisitor<Object> {
     if (node.isCascaded) {
       _writer.print("..");
     } else {
-      _visitNodeWithSuffix(node.target, ".");
+      if (node.target != null) {

[Brian Wilkerson, 2015/04/05 19:55:45]

Seems like it would be cleaner and more consistent with:
  _visitNodeWithSuffix(node.target, node.operator.lexeme);

[Paul Berry, 2015/04/05 20:49:43]

I considered that, but unfortunately it doesn't work because node.operator can
be null (it's null if there's no target).

I suppose I could do this:

if (node.target != null) {
  _visitNodeWithSuffix(node.target, node.operator.lexeme);
}

or this:

_visitNodeWithSuffix(node.target, node.operator == null ? null :
node.operator.lexeme);

But both of those possibilities seem clumsy because they have one if-test in the
caller and an equivalent if-test in the callee.  So I opted to just spell out
the logic explicitly.

Keeping as is for now.  If you have strong feelings I can always do a follow-up
CL.

As a side note, if it weren't for the chicken-and-egg problem this would have
been an excellent opportunity to do:

_visitNodeWithSuffix(node.target, node.operator?.lexeme);

:)

+        node.target.accept(this);
+        _writer.print(node.operator.lexeme);
+      }
     }
     _visitNode(node.methodName);
     _visitNode(node.argumentList);