Document dart2js flags in dartjs -h

R=kathyw@google.com, sra@google.com

Review-Url: https://codereview.chromium.org/2974133002 .

pkg/compiler/lib/src/dart2js.dart

@@ -738,12 +738,34 @@ Supported options:
     `uri` getter for `LibraryMirror`s is mangled in minified mode.
 
   --csp
-    Disables dynamic generation of code in the generated output. This is
+    Disable dynamic generation of code in the generated output. This is
     necessary to satisfy CSP restrictions (see http://www.w3.org/TR/CSP/).
 
   --no-source-maps
     Do not generate a source map file.
 
+  --fast-startup
+    Produce JavaScript that can be parsed more quickly by VMs. This option
+    usually results in larger JavaScript files with faster startup.
+    Note: the dart:mirrors library is not supported with this option.
+
+The following advanced options can help reduce the size of the generated code,
+but they may cause programs to behave unexpectedly if assumptions are not met.
+Only turn on these flags if you have enough test coverage to ensure they are
+safe to use:
+
+  --trust-type-annotations
+    Assume that all types are correct. This option allows the compiler to drop
+    type checks and to rely on local type information for optimizations. Use
+    this option only if you have enough testing to ensure that your program
+    works in strong mode or checked mode.
+
+  --trust-primitives
+    Assume that operations on numbers, strings, and lists have valid inputs.
+    This option allows the compiler to drop runtime checks for those operations.
+    Note: a well-typed program is not guaranteed to have valid inputs. For
+    example, an int index argument may be null or out of range.
+
 The following options are only used for compiler development and may
 be removed in a future version: