Currently, template expressions and statements have their location
recorded relative to the HTML element they are in, with no handle to
absolute location in a source file except for a line/column location.
However, the line/column location is also not entirely accurate, as it
points an entire semantic expression, and not necessarily the start of
an expression recorded by the expression parser.
To support record of the source code expressions originate from, add a
new `sourceSpan` field to `ASTWithSource` that records the absolute byte
offset of an expression within a source code.
Implement part 2 of [refactoring template parsing for
stability](https://hackmd.io/@X3ECPVy-RCuVfba-pnvIpw/BkDUxaW84/%2FMA1oxh6jRXqSmZBcLfYdyw?type=book).
PR Close#31391
Since `goog.getMsg` does not process ICUs (post-processing is required via goog.i18n.MessageFormat, https://google.github.io/closure-library/api/goog.i18n.MessageFormat.html) and placeholder format used for ICUs and regular messages inside `goog.getMsg` are different, the current implementation (that assumed the same placeholder format) needs to be updated. This commit updates placeholder format used inside ICUs from `{$placeholder}` to `{PLACEHOLDER}` to better align with Closure. ICU placeholders (that were left as is prior to this commit) are now replaced with actual values in post-processing step (inside `i18nPostprocess`).
PR Close#31459
Prior to this change i18n block bindings were converted to Expressions right away (once we first access them), when in non-i18n cases we processed them differently: the actual conversion happens at instructions generation. Because of this discrepancy, the output for bindings in i18n blocks was generated incorrectly (with invalid indicies in pipeBindN fns and invalid references to non-existent local variables). Now the bindings processing is unified and i18nExp instructions should contain right bind expressions.
PR Close#28969
