Added informal spec of optional const/new to docs/language/informal.

docs/language/informal/optional-new.md

+# Optional new
+
+Author: eernst@.
+
+Version: 0.1 (2017-08-15)
+
+Status: Under discussion
+
+**This document** is an informal specification of the *optional new* feature.
+**The feature** adds support for omitting the reserved word `new` in instance
+creation expressions.
+
+This feature relies on
+[optional const](https://gist.github.com/eernstg/4f498836e73d5f003928e8bbe1683d68),
+and it is assumed that the reader knows the optional const feature. Otherwise,
+this informal specification is derived from a
+[combined proposal](https://github.com/dart-lang/sdk/blob/master/docs/language/informal/optional-new-const.md)
+which presents optional new and several other features.
+
+## Motivation
+
+In Dart without optional new, the reserved word `new` is present in every
+expression whose evaluation invokes a constructor (except constant
+expressions). These expressions are known as *instance creation expressions*. If
+`new` is removed from such an instance creation expression, the remaining phrase
+is still syntactically correct in almost all cases, and the required grammar
+update that makes them all syntactically correct is exactly the one that is
+specified for
+[optional const](https://gist.github.com/eernstg/4f498836e73d5f003928e8bbe1683d68).
+
+Assuming the grammar update in
+[optional const](https://gist.github.com/eernstg/4f498836e73d5f003928e8bbe1683d68),
+all instance creation expressions can technically omit the `new` because tools
+(compilers, analyzers) are able to parse these expressions, and they are able
+to recognize that they denote instance creations (rather than, say, static
+function invocations), because the part before the left parenthesis is
+statically known to denote a constructor.
+
+For instance, `p.C.foo` may resolve statically to a constructor named `foo` in
+a class `C` imported with prefix `p`. Similarly, `D` may resolve to a class, in
+which case `D(42)` is statically known to be a constructor invocation because
+the other interpretation is statically known to be incorrect (that is, cf.
+section '16.14.3 Unqualified Invocation' in the language specification,
+evaluating `(D)(42)`: `(D)` is an instance of `Type` which is not a function
+type and does not have a method named `call`).
+
+For human readers, it may be helpful to document that a particular expression
+is guaranteed to yield a fresh instance, and this is the most common argument
+why `new` should *not* be omitted. However, Dart already allows instance
+creation expressions to invoke a factory constructor, so Dart developers never
+had any local guarantees that any particular expression would yield a fresh
+object.
+
+Developers may thus prefer to omit `new` in order to obtain more concise code,
+and possibly also in order to achieve greater uniformity among invocations of
+constructors and other invocations, e.g., of static or global functions.
+
+With that in mind, this proposal allows instance creation expressions to omit
+the `new` in all cases, but also preserves the permission to include `new` in
+all cases. It is a matter of style to use `new` in a manner that developers
+find helpful.
+
+## Syntax
+
+The syntax changes associated with this feature are the following:
+
+```
+postfixExpression ::=
+    assignableExpression postfixOperator |
+    constructorInvocation |  // NEW
+    primary selector*
+constructorInvocation ::=  // NEW
+    typeName typeArguments '.' identifier arguments
+assignableExpression ::=
+    SUPER unconditionalAssignableSelector |
+    typeName typeArguments '.' identifier arguments
+        (arguments* assignableSelector)+ |  // NEW
+    identifier |
+    primary (arguments* assignableSelector)+
+```
+
+This grammar update is identical to the grammar update for optional const.
+For more information including a complete grammar, please consult 
+[that specification](https://gist.github.com/eernstg/4f498836e73d5f003928e8bbe1683d68).
+
+## Static analysis
+
+We specify a type directed source code transformation which eliminates the 
+feature. The static analysis proceeds to work on the transformed program.
+
+*Similarly to optional const, this means that the feature is "static semantic
+sugar". We do not specify the dynamic semantics for this feature, because the
+feature is eliminated in this transformation step.*
+
+We need to treat expressions differently in different locations, hence the
+following definition: An expression _e_ is said to *occur in a constant
+context*,
+
+- if _e_ is an immediate subexpression of a constant list literal or a constant
+  map literal.
+- if _e_ is an immediate subexpression of a constant object expression.
+- if _e_ is the initializing expression of a constant variable declaration.
+- if _e_ is the default value of a formal parameter. **[This case is under discussion and may be removed]**
+- if _e_ is an immediate subexpression of an expression which occurs in a
+  constant context.
+
+We define *new/const insertion* as the following transformation:
+
+- if _e_ occurs in a constant context, replace `e` by `const e`.
+- otherwise, replace `e` by `new e`
+
+An expression on one of the following forms must be modified to be or
+contain a `constantObjectExpression` or `newExpression` as described:
+
+With a `postfixExpression` _e_,
+
+- if _e_ is on the form `constructorInvocation`, i.e.,
+  `typeName typeArguments '.' identifier arguments` then perform
+  new/const insertion on _e_.
+- if _e_ is on the form
+  `typeIdentifier arguments` where `typeIdentifier` denotes a class then
+  perform new/const insertion on _e_.
+- if _e_ is on the form
+  `identifier1 '.' identifier2 arguments` where `identifier1` denotes
+  a class and `identifier2` is the name of a named constructor in that class,
+  or `identifier1` denotes a prefix for a library _L_ and `identifier2` denotes
+  a class exported by _L_, perform new/const insertion on _e_.
+-  if _e_ is on the form
+  `identifier1 '.' typeIdentifier '.' identifier2 arguments` where 
+  `identifier1` denotes a library prefix for a library _L_, `typeIdentifier`
+  denotes a class _C_ exported by _L_, and `identifier2` is the name of a named
+  constructor in _C_, perform new/const insertion on _e_.
+
+## Dynamic Semantics
+
+There is no dynamic semantics to specify for this feature because it is
+eliminated by code transformation.
+
+## Interplay with optional const
+
+The optional new and optional const feature can easily be introduced at the same
+time: Just update the grammar as specified for optional const (and mentioned
+again here) and use the program transformation specified in this document. The
+program transformation in this document subsumes the program transformation
+specified for optional const, and hence this will provide support for both
+features.
+
+## Revisions
+
+- 0.1 (2017-08-15) Stand-alone proposal for optional new created, using version
+  0.8 of the combined proposal
+  [optional-new-const.md](https://github.com/dart-lang/sdk/blob/master/docs/language/informal/optional-new-const.md)
+  as the starting point.