Represent implicit 'this' binding by 'super' in AST.

Represent implicit 'this' binding by 'super' in AST.

This makes the implicit initializing assignment to 'this' performed
after a super constructor call explicit in the AST. It removes the
need to handle the special case where a CallExpression behaves like a
AssignmentExpression from various AstVisitor implementations.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/1226123010

Cr-Commit-Position: refs/heads/master@{#29705}