The asm typer would CHECK fail for imports like

var bogus = stdlib.<HeapViewType>

This CL changes this behavior so the typer reports a validation error
instead.

BUG= https://bugs.chromium.org/p/v8/issues/detail?id=4203
TEST=cctest/asmjs/test-asm-typer.cc
LOG=N

Review-Url: https://codereview.chromium.org/2152763002
Cr-Commit-Position: refs/heads/master@{#37777}