Pasting text after a large untokenized text block should not block

[alexdima]

• Download checker.ts from https://github.com/Microsoft/TypeScript/blob/296660a2a077c32a2ed41cb762ef530031e56417/src/compiler/checker.ts
• open the file and quickly select all and go to the bottom of the file
• (tokenization did not have a chance to execute entirely)
• paste
• observe: VS Code is freezing until all the text is tokenized. I suggest we probe for the last tokenized line and if a large body of text needs to be tokenized to support auto indent, we skip auto indenting.

CPU-20170706T101827.cpuprofile.txt

Note: we should do the same for Enter: #21564

[rebornix]

After the fix, the pasting should always work whether the tokenization process finishes or not, but the auto indent adjustment should kick in when the tokenization is done.