VM should treat throw as an expression

[gbracha]

Per issue #587

[DartBot]

This comment was originally written by @mhausner

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Set owner to @mhausner.
Added Started label.

[DartBot]

This comment was originally written by @mhausner

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Checked in at r12928.

One restriction: throw without an expression is only allowed at statement level.

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Added Fixed label.