Add a compiler option to specify strict mode treatment of

input javascript.

The option defaults to "on", but is ignored for language modes
that disallow or explictly require strict mode.

Also removed the isStrict() method from LanguageMode,
since we're moving to separate specification of this for
future language modes.

This is a no-op change.
It is a step toward adding a flag to control strict mode treatment of inputs
independent of input language mode.

Related to #2068

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136870289

src/com/google/javascript/jscomp/CommandLineRunner.java

@@ -1629,7 +1629,7 @@ protected CompilerOptions createOptions() {
     }
 
     options.setPrintSourceAfterEachPass(flags.printSourceAfterEachPass);
-    options.setEmitUseStrict(flags.emitUseStrict && options.getLanguageOut().isStrict());
+    options.setEmitUseStrict(flags.emitUseStrict);
 
     return options;
   }

src/com/google/javascript/jscomp/Compiler.java

@@ -409,7 +409,7 @@ protected void reconcileOptionsWithGuards() {
           options.checkGlobalThisLevel);
     }
 
-    if (options.getLanguageIn().isStrict()) {
+    if (expectStrictModeInput()) {
       options.setWarningLevel(
           DiagnosticGroups.ES5_STRICT,
           CheckLevel.ERROR);
@@ -427,6 +427,21 @@ protected void reconcileOptionsWithGuards() {
     }
   }
 
+  private boolean expectStrictModeInput() {
+    switch (options.getLanguageIn()) {
+      case ECMASCRIPT3:
+      case ECMASCRIPT5:
+      case ECMASCRIPT6:
+        return false;
+      case ECMASCRIPT5_STRICT:
+      case ECMASCRIPT6_STRICT:
+      case ECMASCRIPT6_TYPED:
+        return true;
+      default:
+        return options.isStrictModeInput();
+    }
+  }
+
   /**
    * Initializes the instance state needed for a compile job.
    */
@@ -1980,11 +1995,21 @@ private String toSource(Node n, SourceMap sourceMap, boolean firstOutput) {
     builder.setCompilerOptions(options);
     builder.setSourceMap(sourceMap);
     builder.setTagAsExterns(firstOutput && options.shouldGenerateTypedExterns());
-    builder.setTagAsStrict(
-        firstOutput && options.isEmitUseStrict() && options.getLanguageOut().isStrict());
+    builder.setTagAsStrict(firstOutput && shouldEmitUseStrict());
     return builder.build();
   }
 
+  private boolean shouldEmitUseStrict() {
+    switch (options.getLanguageOut()) {
+      case ECMASCRIPT3:
+      case ECMASCRIPT5:
+      case ECMASCRIPT6:
+        return false;
+      default:
+        return options.isEmitUseStrict();
+    }
+  }
+
   /**
    * Stores a buffer of text to which more can be appended.  This is just like a
    * StringBuilder except that we also track the number of lines.

src/com/google/javascript/jscomp/CompilerOptions.java

@@ -1042,6 +1042,13 @@ public void setWrapGoogModulesForWhitespaceOnly(boolean enable) {
    */
   boolean printConfig = false;
 
+  /**
+   * Are the input files written for strict mode?
+   *
+   * <p>Ignored for language modes that do not support strict mode.
+   */
+  private boolean isStrictModeInput = true;
+
   /**
    * Should the compiler print its configuration options to stderr when they are initialized?
    *
@@ -2864,21 +2871,6 @@ public static enum LanguageMode {
      */
     NO_TRANSPILE;
 
-    /** Whether this is a "strict mode" language. */
-    public boolean isStrict() {
-      Preconditions.checkState(this != NO_TRANSPILE);
-      switch (this) {
-        case ECMASCRIPT7:
-        case ECMASCRIPT8:
-        case ECMASCRIPT5_STRICT:
-        case ECMASCRIPT6_STRICT:
-        case ECMASCRIPT6_TYPED:
-          return true;
-        default:
-          return false;
-      }
-    }
-
     /** Whether this is ECMAScript 5 or higher. */
     public boolean isEs5OrHigher() {
       Preconditions.checkState(this != NO_TRANSPILE);
@@ -3157,4 +3149,13 @@ boolean isExplicitlyOn() {
       return this == TRUE || this == ON;
     }
   }
+
+  public boolean isStrictModeInput() {
+    return isStrictModeInput;
+  }
+
+  public CompilerOptions setStrictModeInput(boolean isStrictModeInput) {
+    this.isStrictModeInput = isStrictModeInput;
+    return this;
+  }
 }