Enable '--strictNullChecks' (#22088)

* 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

src/services/patternMatcher.ts

@@ -119,7 +119,7 @@ namespace ts {
         // First, check that the last part of the dot separated pattern matches the name of the
         // candidate.  If not, then there's no point in proceeding and doing the more
         // expensive work.
-        const candidateMatch = matchSegment(candidate, lastOrUndefined(dotSeparatedSegments), stringToWordSpans);
+        const candidateMatch = matchSegment(candidate, last(dotSeparatedSegments), stringToWordSpans);
         if (!candidateMatch) {
             return undefined;
         }
@@ -203,7 +203,7 @@ namespace ts {
         }
     }
 
-    function matchSegment(candidate: string, segment: Segment, stringToWordSpans: Map<TextSpan[]>): PatternMatch {
+    function matchSegment(candidate: string, segment: Segment, stringToWordSpans: Map<TextSpan[]>): PatternMatch | undefined {
         // First check if the segment matches as is.  This is also useful if the segment contains
         // characters we would normally strip when splitting into parts that we also may want to
         // match in the candidate.  For example if the segment is "@int" and the candidate is
@@ -260,7 +260,7 @@ namespace ts {
         return bestMatch;
     }
 
-    function betterMatch(a: PatternMatch | undefined, b: PatternMatch | undefined): PatternMatch {
+    function betterMatch(a: PatternMatch | undefined, b: PatternMatch | undefined): PatternMatch | undefined {
         return min(a, b, compareMatches);
     }
     function compareMatches(a: PatternMatch | undefined, b: PatternMatch | undefined): Comparison {
@@ -287,8 +287,8 @@ namespace ts {
 
         let currentCandidate = 0;
         let currentChunkSpan = 0;
-        let firstMatch: number;
-        let contiguous: boolean;
+        let firstMatch: number | undefined;
+        let contiguous: boolean | undefined;
 
         while (true) {
             // Let's consider our termination cases