Underline assignment correctly in error message

[liufengyun]

Underline assignment correctly in error messages

It also reduces the warning in PatternMatcher in bootstrapping.

[olhotak]

Did reverting the expensive eager promotion of field initializers not affect any tests? What code example was eager promotion of fields motivated by?

[liufengyun]

Did reverting the expensive eager promotion of field initializers not affect any tests? What code example was eager promotion of fields motivated by?

It's motivated by the code in patternMatcher. The latest commit solves the problem differently. I'll minimize the code from patternMatcher and add it as a test.