Harvest from pub. Mostly a straight copy, except:

README.md

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+Handles version numbers and version constraints in the same way that [pub][]
+does. The semantics here very closely follow the [Semantic Versioning][semver]
+spec. It differs from semver in a few corner cases:
+
+ *  **Version ordering does take build suffixes into account.** This is unlike
+    semver 2.0.0 but like earlier versions of semver. Version `1.2.3+1` is
+    considered a lower number than `1.2.3+2`.
+
+    Since a package may have published multiple versions that differ only by
+    build suffix, pub still has to pick one of them *somehow*. Semver leaves
+    that issue unresolved, so we just say that build numbers are sorted like
+    pre-release suffixes.
+
+ *  **Pre-release versions are excluded from most max ranges.** Let's say a
+    user is depending on "foo" with constraint `>=1.0.0 <2.0.0` and that "foo"
+    has published these versions:
+
+     *  `1.0.0`
+     *  `1.1.0`
+     *  `1.2.0`
+     *  `2.0.0-alpha`
+     *  `2.0.0-beta`
+     *  `2.0.0`
+     *  `2.1.0`
+
+    Versions `2.0.0` and `2.1.0` are excluded by the constraint since neither
+    matches `<2.0.0`. However, since semver specifies that pre-release versions
+    are lower than the non-prerelease version (i.e. `2.0.0-beta < 2.0.0`, then
+    the `<2.0.0` constraint does technically allow those.
+
+    But that's almost never what the user wants. If their package doesn't work
+    with foo `2.0.0`, it's certainly not likely to work with experimental,
+    unstable versions of `2.0.0`'s API, which is what pre-release versions
+    represent.
+
+    To handle that, `<` version ranges to not allow pre-release versions of the
+    maximum unless the max is itself a pre-release. In other words, a `<2.0.0`
+    constraint will prohibit not just `2.0.0` but any pre-release of `2.0.0`.
+    However, `<2.0.0-beta` will exclude `2.0.0-beta` but allow `2.0.0-alpha`.
+
+ *  **Pre-release versions are avoided when possible.** The above case
+    handles pre-release versions at the top of the range, but what about in
+    the middle? What if "foo" has these versions:
+
+     *  `1.0.0`
+     *  `1.2.0-alpha`
+     *  `1.2.0`
+     *  `1.3.0-experimental`
+
+    When a number of versions are valid, pub chooses the best one where "best"
+    usually means "highest numbered". That follows the user's intuition that,
+    all else being equal, they want the latest and greatest. Here, that would
+    mean `1.3.0-experimental`. However, most users don't want to use unstable
+    versions of their dependencies.
+
+    We want pre-releases to be explicitly opt-in so that package consumers
+    don't get unpleasant surprises and so that package maintainers are free to
+    put out pre-releases and get feedback without dragging all of their users
+    onto the bleeding edge.
+
+    To accommodate that, when pub is choosing a version, it uses *priority*
+    order which is different from strict comparison ordering. Any stable
+    version is considered higher priority than any unstable version. The above
+    versions, in priority order, are:
+
+     *  `1.2.0-alpha`
+     *  `1.3.0-experimental`
+     *  `1.0.0`
+     *  `1.2.0`
+
+    This ensures that users only end up with an unstable version when there are
+    no alternatives. Usually this means they've picked a constraint that
+    specifically selects that unstable version -- they've deliberately opted
+    into it.
+
+[pub]: http://pub.dartlang.org/
+[semver]: http://semver.org/