Add deprication for concatenating string literals with + in analyzer

[DartBot]

This issue was originally filed by zundel@google.com

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The new way of concatenating string literals is "foo" "bar" to make "foobar"
instead of using the + operator. Add a deprecation now to become a compile time error later.

[kasperl]

Added Triaged label.

[kasperl]

Removed Area-Compiler label.
Added Area-Analyzer label.

[danrubel]

Added this to the M1 milestone.

[DartBot]

This comment was originally written by zundel@google.com

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This will be a common mistake for Dart newbies and was easy to add.

https://chromiumcodereview.appspot.com/10540029

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Set owner to zundel@google.com.
Removed Type-Defect label.
Added Type-Enhancement label.

[DartBot]

This comment was originally written by zundel@google.com

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r8350

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Added Fixed label.