AstGraphBuilder puts a constant context in from the beginning. Also fix bug in merging contexts in environment. R=mstarzinger@chromium.org BUG= Review URL: https://codereview.chromium.org/934293002 Cr-Commit-Position: refs/heads/master@{#26745}
R=mstarzinger@chromium.org BUG= Review URL: https://codereview.chromium.org/921083004 Cr-Commit-Position: refs/heads/master@{#26705}