Bug 1102131 - Part 1: Fix spelling of "LetExpresion". r=shu

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extra : rebase_source : d5aeb746550df4aed30cad5c4a795998ed5568ff
Bug 1102131 - Part 2: Collect telemetry about deprecated let blocks and expressions. r=shu

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extra : rebase_source : 9e0db2c7a2f90545d27e78d46a334dbc27554209
Bug 1102131 - Part 3: Log console warnings for deprecated let expressions. r=shu

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extra : rebase_source : 2b979c45d9d35a259905dd242d2efe6148f13ed2
Bug 1102131 - Part 5: Remove deprecated let blocks and expressions in toolkit. r=Mossop

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extra : rebase_source : 3581ec35850edb35c921a0d86ddae1391f588135
Bug 1102131 - Part 6: Remove deprecated let block in DownloadsCommon.jsm. r=mak

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extra : rebase_source : d8b36142d84edf02fd88cb303b7fa61140bccc48