fix(compiler): attribute case when parsing HTML in IE9

[marclaval]

Investigating a test failure in IE9, I found an interesting issue about parsing attributes.
Consider the following code snippet:

myElem = document.createElement('div');
myElem.innerHTML = '<input type="text" ng-control="min" maxlength="3" ABC="4" efGH="5"></input>';
myElem.outerHTML

Output in Chrome:
<div><input type="text" ng-control="min" maxlength="3" abc="4" efgh="5"></div>
Output in Firefox:
<div><input type="text" ng-control="min" maxlength="3" ABC="4" efGH="5" /></div>
Output in IE9:
<div><input maxLength="3" type="text" ng-control="min" efGH="5" ABC="4"></div>

As you can see, IE9 and Firefox keep the case, and IE9 even modifies it in the particular case of maxlength.
Hence the proposal to lower case all attributes names to harmonize the behaviors.

[marclaval]

@IgorMinar Sauce Labs is back to green :)

[vicb]

attrName.toLowerCase() should work in both Dart & JS.(we should probably remove the methods (lower & upper) from the Wrapper).

Should you add a comment here ?

[tbosch]

Looks good

[IgorMinar]

lgtm

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