Perf: Move multiline xml text encoding out of domToText

[eanders-ms]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

A complex regular expression in Blockly.Xml.domToText was running on every save, causing performance problems on large projects. The regexp progressively replaces all \n with 
 in content fields, and was added along with FieldMultilineInput, in #2663.

Proposed Changes

Override toXml and fromXml on FieldMultilineInput and do the conversion there. This simplifies the regexp, and it only runs when there is multiline content in the program, improving performance - especially on large programs.

Performance Comparison

Testing with this MakeCode Arcade game: https://arcade.makecode.com/74548-67333-76127-33508

Time spent in Blockly.Xml.domToText:

Before	After
10s	250ms

Resolves

Resolves #4030

Test Coverage

I tested this change in playground.html. Steps I followed:

1. Add a few paragraph blocks and populate them with multiline content.
2. Export to Xml. Inspect the Xml to verify content encoding.
3. Delete all blocks.
4. Import from Xml. Verify (visually) that the content looks correct.

Also: In the dev console, set breakpoints in the toXml and fromXml methods of FieldMultilineInput to verify they're getting called at the right times.

Tested on:

• Desktop Chrome

Additional information

Of note: Another document-wide regexp has appeared in domToText, and will likely become a performance issue. You may want to fix it: https://github.com/google/blockly/blob/ba68081d8f0f8e81d76baf370716d5955bae82a1/core/xml.js#L327

I can see how domToText would be a tempting place to fixup formatting issues. It might be worth adding a comment in that method stating: "For performance reasons, don't run regular expressions on the whole document."

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[samelhusseini]

LGTM pending CLA. Thank you!

[eanders-ms]

@googlebot I signed it!

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[samelhusseini]

Thanks!