Fix analyzer error

Analyzer was complaining that final field was not set.  The other constructor
does set, so doing so here as well.  We may be able to remove this flag soon,
but I see downstream uses.

TBR=jmesserly@google.com

Review URL: https://codereview.chromium.org/2382463002 .

pkg/dev_compiler/lib/src/analyzer/context.dart

@@ -51,6 +51,7 @@ class AnalyzerOptions {
       String dartSdkPath,
       this.dartSdkSummaryPath,
       this.customUrlMappings: const {},
+      this.packageRoot: null,
       this.packagePaths: const []})
       : dartSdkPath = dartSdkPath ?? getSdkDir().path;