* remove invalid assertion
* fix invalid array spread on possibly undefined value
* only add unique files to extendedSourceFiles, preventing the array from growing infinitely
* Enable '--strictNullChecks'
* Fix API baselines
* Make sys.getEnvironmentVariable non-nullable
* make properties optional instead of using `| undefined` in thier type
* reportDiagnostics should be required
* Declare firstAccessor as non-nullable
* Make `some` a type guard
* Fix `getEnvironmentVariable` definition in tests
* Pretend transformFlags are always defined
* Fix one more use of sys.getEnvironmentVariable
* `requiredResponse` accepts undefined, remove assertions
* Mark optional properties as optional instead of using `| undefined`
* Mark optional properties as optional instead of using ` | undefined`
* Remove unnecessary null assertions
* Put the bang on the declaration instead of every use
* Make `createMapFromTemplate` require a parameter
* Mark `EmitResult.emittedFiles` and `EmitResult.sourceMaps` as optional
* Plumb through undefined in emitLsit and EmitExpressionList
* `ElementAccessExpression.argumentExpression` can not be `undefined`
* Add overloads for `writeTokenText`
* Make `shouldWriteSeparatingLineTerminator` argument non-nullable
* Make `synthesizedNodeStartsOnNewLine` argument required
* `PropertyAssignment.initializer` cannot be undefined
* Use one `!` at declaration site instead of on every use site
* Capture host in a constant and avoid null assertions
* Remove few more unused assertions
* Update baselines
* Use parameter defaults
* Update baselines
* Fix lint
* Make Symbol#valueDeclaration and Symbol#declarations non-optional to reduce assertions
* Make Node#symbol and Type#symbol non-optional to reduce assertions
* Make `flags` non-nullable to reduce assertions
* Convert some asserts to type guards
* Make `isNonLocalAlias` a type guard
* Add overload for `getSymbolOfNode` for `Declaration`
* Some more `getSymbolOfNode` changes
* Push undefined suppression into `typeToTypeNodeHelper`
* `NodeBuilderContext.tracker` is never `undefined`
* use `Debug.assertDefined`
* Remove unnecessary tag
* Mark `LiteralType.freshType` and `LiteralTupe.regularType` as required
