JSCompiler does not properly handle escaped backslash in tagged template literal

Using a tagged template literal with an escaped backslash causes both backslashes to be passed to the template function, instead of just the single backslash. The raw property contains both backslashes as expected.

For example
```
test`\\.`
compiled to: test({0: "\\\\.", raw: "\\\\."})
expected: test({0: "\\.", raw: "\\\\."})
```
However, other escape sequences work as expected
```
test`\n.`
compiled to: test({0: "\n.", raw: "\\n."})
```

https://closure-compiler-debugger.appspot.com/#input0%3Dfunction%2520test(...args)%2520%257B%250A%2520%2520console.log(args)%253B%250A%257D%250A%250Atest%2560%255C%255C.%2560%250Atest%2560%255Cn.%2560%26input1%26conformanceConfig%26externs%26refasterjs-template%26includeDefaultExterns%3D1%26CHECK_SYMBOLS%3D1%26MISSING_PROPERTIES%3D1%26TRANSPILE%3D1%26CHECK_TYPES%3D1%26CLOSURE_PASS%3D1%26PRESERVE_TYPE_ANNOTATIONS%3D1%26PRETTY_PRINT%3D1