g-component custom events can be handled in wrong scope about polymer HOT 4 CLOSED

Fixed by a88671e

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The issue is still happening in shim ShadowDOM.

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Steve made issues/pending/shimShadowComponentEvents test case.

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The problem arises because we are trying to walk into the shadow DOM from composed DOM.

Natively, a node exists only in one local DOM and the composed DOM is virtual (and can be ignored). Shim requires that a node exists only in composed DOM, so placeholders are used for other trees (Changelings).

Naively, node.changeling is used to locate the shadow DOM subtree where node is intended to exist. Unfortunately, multiple placeholders are used to implement projections and compositions, so node.changeling often refers to the wrong subtree.

I've attempted to solve this problem by:

(1) making sure that node.changeling is only set once (i.e. cache the initial position only)
(2) setting lightDOM.host
(3) coding the controller tree climber to use lightDOM.host as if it were parentNode

I may have missed a wrinkle, or there could be negative side-effects, which is why I wanted to document it here.

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