Add -keep command line option to JS DCE tool

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JSDceArguments.kt

@@ -26,6 +26,13 @@ class K2JSDceArguments : CommonToolArguments() {
     @JvmField
     var outputDirectory: String? = null
 
+    @field:Argument(
+            value = "-keep",
+            valueDescription = "<fully.qualified.name[,]>",
+            description = "List of fully-qualified names of declarations that shouldn't be eliminated")
+    @JvmField
+    var declarationsToKeep: Array<String>? = null
+
     @field:GradleOption(DefaultValues.BooleanFalseDefault::class)
     @field:Argument(value = "-Xprint-reachability-info", description = "Print declarations marked as reachable")
     @JvmField

compiler/cli/src/org/jetbrains/kotlin/cli/js/dce/K2JSDce.kt

@@ -49,7 +49,9 @@ class K2JSDce : CLITool<K2JSDceArguments>() {
             return ExitCode.COMPILATION_ERROR
         }
 
-        val dceResult = DeadCodeElimination.run(files, emptySet()) {
+        val includedDeclarations = arguments.declarationsToKeep.orEmpty().toSet()
+
+        val dceResult = DeadCodeElimination.run(files, includedDeclarations) {
             messageCollector.report(CompilerMessageSeverity.LOGGING, it)
         }
         val nodes = dceResult.reachableNodes